With each generation of metal oxide semiconductor (MOS) integrated circuits (IC), device dimensions have been continuously scaled down to provide for high-density and high-performance, such as high speed and low power consumption requirements. Unfortunately, field effect semiconductor devices produce an output signal that is proportional to the width of the channel, such that scaling reduces their output. This effect has generally been compensated for by decreasing the thickness of the gate dielectric, thus bringing the gate in closer proximity to the channel and enhancing the field effect, which thereby increases the drive current. Therefore, it has become increasingly important to provide extremely thin, reliable and low-defect gate dielectrics for improving device performance.
For decades, a thermal silicon oxide, SiO2 has been the primary gate dielectric, because it is compatible with the underlying silicon substrate, and its fabrication process is relatively simple. However, because the silicon oxide gate dielectric has a relatively low dielectric constant (k), 3.9, further scaling down of silicon oxide gate dielectric thickness to less than 10 Å has become more and more difficult, especially due to gate-to-channel leakage current through the thin silicon oxide gate dielectric.
This leads to consideration of alternative dielectric materials, which can be formed in a thicker layer than silicon oxide, but still produce the same or better device performance. This performance can be expressed as “equivalent oxide thickness (EOT)”. Although the alternative dielectric material layer may be thicker than a comparative silicon oxide layer, it has the equivalent effect of a much thinner layer of a silicon oxide layer.
To this end, high-k metal oxide materials have been proposed as the alternative dielectric materials for gate or capacitor dielectrics. Group 4-containing precursors may also be used by themselves or combined with other metal-containing precursors to make high dielectric constant and/or ferroelectric oxide thin films such as, for example, Pb(Zr,Ti)O3 or (Ba,Si)(Zr,Ti)O3. Because the dielectric constant of metal oxide materials can be made greater than that of the silicon oxide, a thicker metal oxide layer having a similar EOT can be deposited. As a result, the semiconductor industry requires Group 4 precursors, such as, for example, titanium-containing, zirconium-containing, and hafnium-containing precursors and combinations thereof, to be able to deposit metal-containing films such as, but not limited to, oxide, nitride, silicate or combinations thereof on substrates, such as metal nitride or silicon.
Unfortunately, the use of high-k metal oxide materials presents several problems, when using traditional substrate materials, such as silicon. The silicon can react with the high-k metal oxide or be oxidized during deposition of the high-k metal oxide or subsequent thermal processes, thereby forming an interface layer of silicon oxide. This increases the equivalent oxide thickness, thereby degrading device performance. Further, an interface trap density between the high-k metal oxide layer and the silicon substrate is increased. Thus, the channel mobility of the carriers is reduced. This reduces the on/off current ratio of the MOS transistor, thereby degrading its switching characteristics. Also, the high-k metal oxide layer, such as a hafnium oxide (HfO2) layer or a zirconium oxide (ZrO2) layer, has a relatively low crystallization temperature and is thermally unstable. Thus, the metal oxide layer can be easily crystallized during a subsequent thermal annealing process for activating the dopants injected into source/drain regions. This can form grain boundaries in the metal oxide layer, through which current can pass. As the surface roughness of the metal oxide layer increases, the leakage current characteristics may deteriorate. Further, the crystallization of the high-k metal oxide layer undesirably affects a subsequent alignment process due to irregular reflection of the light on an alignment key having the rough surface.
Group 4 metal-containing films can be deposited using chemical vapor deposition (CVD) and/or atomic layer deposition (ALD) processes. In a traditional CVD process the vapors of one or more volatile precursors are introduced into a chemical vapor deposition reactor loaded with a semi-fabricated substrate, which has been pre-heated to the temperature above the thermal decomposition of at least one of the precursors. The rate of film growth is determined by the rate of reaction between the reactants on the surface, and the film growth continues, as long as reactant vapors are present in the vapor phase. On the other hand, in an atomic layer deposition (ALD) process, reactants are introduced into an ALD reactor sequentially, thus avoiding any gas phase reactions between the reactants. A typical cycle of ALD processes for deposition metal oxide films includes: 1) introducing enough vapors of a metal containing precursor to the ALD chamber to allow the precursor to chemically adsorb on the surface until the whole surface area is covered; 2) purging the ALD chamber with inert gas to remove any by-products as well as unreacted precursors; 3) introducing an oxidizer to react with the precursor adsorbed on the surface; 4) purging away any unreacted oxidizer and any reaction by-products. The cycle is repeated until a desired thickness is achieved. An ideal ALD process is self-limiting, i.e. the substrate surface is saturated with a reactant during its introduction and the film growth stops even though large excess of precursors are present in the gas phase. Therefore, ALD provides multiple advantages over CVD for deposition of highly conformal films on complex surfaces, such as deep trenches and other stepped structures
The balance between good thermal stability of ALD precursors and the ability of ALD precursors to chemisorb on the substrate surface is very important for producing thin, conformal films of high K dielectric metal oxides. It is desirable that the precursors are thermally stable during vapor delivery in order to avoid premature decomposition of the precursor, before it reaches the vapor deposition chamber during processing. Premature decomposition of the precursor, not only results in undesirable accumulation of side products that would clog fluid flow conduits of the deposition apparatus, but also may cause undesirable variations in composition or as well as dielectric constant and/or ferroelectric properties of the deposited metal oxide thin film.
A number of various delivery systems have been developed for the delivery of precursors to CVD or ALD reactors. For example, in direct liquid injection (DLI) method a liquid precursor or a solution of a precursor in a solvent is delivered to a heated vaporization system, whereby the liquid composition is transferred from the liquid phase into the gas phase. Advanced liquid metering of the precursor to the vaporizer provides accurate, stable control of precursor delivery rate. However, it is critical during the vaporization process that the precursor structure is maintained and decomposition is eliminated. Another method, which is already widely used in semiconductor industry for delivery of metal organic precursors, is based on conventional bubbler technology, where inert gas is bubbled through a neat liquid or a molten precursor at elevated temperature. Typically, precursors have low vapor pressure and have to be heated to 100-200° C. to deliver enough precursor vapors to the deposition reactor by the bubbling method. Solid precursors delivered from their molten phase may plug the lines during multiple cooling/heating cycles. It is desired that precursors are liquids or solids with melting point significantly lower than the bubbler temperature. Products of thermal decomposition may also plug delivery lines and affect the delivery rate of precursors. Extended periods of time at the bubbler temperatures may also cause thermal decomposition of the precursors. The precursors may also react with traces of moisture and oxygen introduced to the bubbler during multiple deposition cycles. It is highly desirable that the precursors maintain their chemical identity over time during storage and delivery. Any change in chemical composition of a Group 4 precursor is deleterious, because it may impair the ability of the vapor deposition process to achieve constant delivery and film growth.
Prior art in the field of the present invention includes: U.S. Pat. No. 6,603,033; Chem. Vap. Deposition, 9, 295 (2003); J. of Less Common Metals, 3, 253 (1961); J. Am. Chem. Soc. 79, p 4344-4348 (1957); Journal of the Chemical Society A: Inorganic, Physical, and Theoretical Chemistry: 904-907 (1970); Chemical Communications 10(14): 1610-1611 (2004); Journal of Materials Chemistry 14, 3231-3238 (2004); Chemical Vapor Deposition 12, 172-180 (2006); JP2007197804A; JP10114781A; WO1984003042A1; JP2822946B2; U.S. Pat. No. 6,562,990B; WO9640690; US2010/0018439A; and Applicants' co-pending application US2007/0248754A1, U.S. Ser. No. 11/945,678 filed on Nov. 27, 2007, Applicants' co-pending application U.S. Ser. No. 12/266,806 which was filed on Nov. 11, 2008, Applicants' co-pending application US 2010/0143607 A1, U.S. Ser. No. 12/629,416 filed on Dec. 2, 2009, or Applicants' U.S. Pat. No. 7,691,984, U.S. Pat. No. 7,723,493.
As previously discussed, the Group 4 precursors in the prior art are mostly solid and have relatively low vapor pressure (e.g., 0.5 torr or below at the delivery temperature). Of a few Group 4 precursors that are in liquid form in the prior art, these precursors are not thermally stable at temperatures greater than 150° C., thus causing delivery or process issues during semiconductor manufacturing, which can include, but are not limited to, lower ALD process window, clogging of the delivery lines between the source container and reactor, and particle deposition on the wafers.
Accordingly, there is a need to develop Group 4 precursors, preferably liquid Group 4 precursors, which exhibit at least one of the following properties: lower molecular weight (e.g., 500 m.u. or below), lower melting point (e.g., 60° C. or below), high vapor pressure (e.g., 0.5 torr or greater). Group 4 precursors having high ALD thermal window (e.g., 300° C. and above) as well as high ALD growth rate (e.g. above 0.3 A/cycle) are also needed. There is also a need to develop Group 4 precursors which are thermally stable and which maintain their chemical composition during storage and delivery.